The present invention relates to a method for accurately matching skin colors under diverse illumination conditions, and to equipment for carrying out such a procedure. More particularly, but not exclusively, it relates to a method and associated equipment for matching the appearance of a prosthesis to a natural skin tone of a user.
Systematic color matching methods are known from many fields, such as paint formulation and plastics pigmentation. Generally speaking, a device such as a colorimeter is used to measure a color of a sample, and a pigment or pigments are selected to produce substantially the same color in the dry paint, compounded polymer, and so forth.
Any color can be defined in terms of three independent co-ordinates, and several alternative co-ordinate systems are in use. Probably the most commonly used is the CIE L*a*b* system, in which a color is assigned a brightness value (L*), a value on a red-green axis (a*) and a value on a blue-yellow axis (b*). An alternative is the CIE L*c*h° system, in which a color is described using a brightness value (L*), a chroma value (c*—in general terms, an intensity of color) and a hue angle (h°—in general terms, which color it is—yellow, orange, purple and so forth).
For color matching in the CIEL*a*b* system, an overall measure of color difference ΔE* is calculated from the individual differences in the three coordinates, ΔL*, Δa* and Δb*, according to the formula:ΔE*=√[(ΔL*)2+(Δa*)2+(Δb*)2]
A skilled color matcher may be able to distinguish between two samples having a ΔE* of around 0.5. For most practical purposes, a ΔE* of 1.0 or below may be considered a visual match. A corresponding overall measure of color difference may be calculated from the respective CIEL*c*h° coordinates, allowing use of the CMC color tolerance system, developed by the Color Measurement Committee of the Society of Dyers and Colorists. This uses a weighted formula which has been found to correlate with results from human color matchers slightly better than the simple ΔE* formula, above. A further color difference measure may be calculated by following the CIE94 system, developed by the Commission Internationale de l'Eclairage. This, too, incorporates weighting factors to produce a better correlation with human results. The CMC and CIE94 color differences should each be 1.0 or below for a match.
Such single point measurements are however not always sufficient. A standard colorimeter is a tristimulus device, making measurements at three wavelengths only (generally a “red”, a “green” and a “blue” wavelength), which are mathematically converted into L*a*b* values or other co-ordinate system of choice.
However, the appearance of a surface in practice depends on the illumination under which it is viewed. Paint color matching is conventionally performed under a standard “north light”, equivalent to north-facing daylight at noon, and a paint colorimeter is hence adapted to simulate this standard illuminant. There are other standard illuminants, for example corresponding generally to incandescent lighting and to fluorescent lighting. A color match produced under a given standard illuminant will only be strictly valid under that illuminant, and not necessarily under others.
The effect whereby two surfaces appear to have the same color under some illumination conditions, but are visibly different under others, is known as metamerism. This is caused by differences in how pigments absorb and reflect light across the whole visible spectrum. Thus, the curves for two pigments (or pigment blends) may differ in a particular wavelength range but be similar elsewhere. Viewed under an illuminant with a low intensity in this wavelength range, the two pigments will appear the same, but under an illuminant with a significant intensity in this range, they will appear substantially different. This effect cannot be measured by a simple tristimulus colorimeter or the like, nor characterised by a small number of coordinate values.
Metamerism is a particular problem when matching skin tones, and especially when matching a prosthetic device to a skin tone of its wearer. While artificial limbs and the like are now available that comprise a silicone plastics material approximating to the wearer's skin color, these are only matched under a standard illuminant, and any visual checking is likely to take place under artificial lighting. The human visual system is particularly sensitised to differences in skin tones, so even small metameric effects will be noticed. A prosthesis that is clearly a prosthesis, for example because it does not match the wearer's skin tone in daylight, is not fully acceptable to the wearer.
As a result, technicians performing color matches for prosthetics have not widely adopted color measurement techniques, and many continue to trust to their experience and mix pigments “by eye”. However, this may require a lengthy process of trial and error, and fails to address the problem of metamerism.
While this problem is significant with pale, Caucasian skin tones, it is believed to be even greater with darker skins, particularly Asian and Afro-Caribbean skin colors. Even experienced technicians can have problems making an initial match to non-Caucasian skin tones, even before issues of metamerism arise. As well as conventional prostheses, camouflaging treatments to conceal the effects of skin conditions such as vitiligo will be more important for darker skin tones.
It is hence an object of the present invention to provide a method for matching skin colors, particularly for prostheses and other medical applications, that obviates the problems with metamerism referred to above and permits more rapid and accurate pigment blend formulation than hitherto. It is also an object of the present invention to provide apparatus for carrying out the above method, and to provide palettes of selected pigments for matching skin colors, including non-Caucasian skin colors.